Current automotive engine control systems include an air-fuel ratio control system having an electromechanical carburetor in which a solenoid is energized according to a command signal and positions a metering valve to control the carburetor fuel flow. The carburetor mixes the fuel with air, and a sensor measures the air-fuel ratio of the mixture. The output of the sensor then adjusts the command signal to provide the fuel flow which will achieve the air-fuel ratio desired.
Current automotive engine control systems also include a fuel vapor storage canister which captures fuel vapor vented from the vehicle fuel tank. During engine operation, air is drawn through the canister to purge the captured fuel vapor from the canister into the engine induction system. During initial operation, the purge flow from the canister may have a high concentration of fuel vapor; after prolonged operation, however, the purge flow from the canister will have a low concentration of fuel vapor.
It will be appreciated that if the carburetor is commanded to minimize fuel flow at a time when the purge flow has a high concentration of fuel vapor, the mixture delivered to the engine may not have the desired air-fuel ratio. Similarly, if the carburetor is commanded to maximize fuel flow at a time when the purge flow has a low concentration of fuel vapor, the desired air-fuel ratio may not be achieved.